The plantUML code below:
actor Smith
box "A box" #LightBlue
participant foo
participant bar
end box
participant baz
Smith -> foo: call foo
alt if something
foo -> bar: do something
else something else
foo -> baz: do something else
end
generate this image:
As you can see, the backgroud of the actors box is above the grouping message (even if the grouping message have a background color).
The diagram could be on top level for a better visibility.
